Osteoporosis is a bone wasting disease that afflicts about 20 million Americans. Osteoporotic fractures occur when bone density (BD) falls below the fracture threshold, a change dependent upon the peak bone density achieved by young adulthood and the subsequent net bone loss after the menopause. Up to 70% of the variation in peak BD within is inheritable. Recognizing that it is important to identify genes responsible for peak BD, the applicants initiated studies in genetically-defined inbred strains of mice. This led to discovery of a genetic model with two inbred mouse strains, C57BL/6J and C3H/HeJ, with highly significant differences in vertebral (11%), tibial (34%), and femoral (54%) BD. By using a combination of pQCT, serum and urine biochemical assays of bone formation (BF) and bone resorption (BR), and histomorphometry, they have obtained the following preliminary data. The C3H/HeJ mice (highest BD) differed from the C57BL/6J mice in the following manners: 1) reduced medullary cavity volume and increased cortical thickness; 2) increased metaphyseal trabecular BD, indicating interstrain differences occur at the endosteum and at trabecular bone: 3) decreased serum osteocalcin, serum skeletal alkaline phosphatase, and urine crosslinks/creatinine at 2 months of age; and 4) decreased osteoclast number at both endosteal cortex and trabecular bone at 2 months of age, suggesting that at this time point, decreased BR contributed to the interstrain difference in BD. Based on this preliminary data, they have advanced two hypotheses: 1) the interstrain difference in BD is determined by a fixed number of genes that can be mapped; and 2) the interstrain difference in BD is a consequence of gene effects on endosteal/trabecular BF or endosteal/trabecular BR, or both. To test the first hypothesis, a combination of genetic crosses and molecular analytic approaches will be applied to: 1) intercross F2 progeny for quantitative trait loci analyses (QTL) with the C57BL/6J and C3H/HeJ strains, plus recombinant inbred (RI) strain analyses using the BXH RI strain set derived from C57BL/6J and C3H/HeJ progenitors; and 2) recombinant congenic (RC) strains plus backcrosses of RC strains with C57BL/6J. Correlation of BD phenotypes with segregating DNA polymorphisms will establish genetic linkage, estimate the number of genes involved with interstrain bone density differences, genetically order bone regulatory genes with major and important modifier effects, define mode of inheritance for each gene, and evaluate parent-of-origin effects on BD. To test the second hypothesis, longitudinal studies will be conducted during development of peak BD in the two inbred mouse strains, applying methodologies for BD, bone histomorphometry, and bone biochemical assays. The data obtained will be analyzed: 1) to quantitatively describe the BF and BR mechanisms that account for the increase in BD within each mouse strain; and 2) to determine the differences in BF and BR that account for the difference in peak BD between the mouse strains. Ultimately, the applicants propose to correlate the phenotypic modeling mechanisms disclosed by their dynamic studies of bone modeling with the genes mapped in the first Aim.